Conventionally, automobile wheel bearing devices support wheels that rotatably bear wheel hubs that mount the wheels by using rolling bearings. They are used for driving wheels or driven wheels. For structural reasons, inner-ring rotation systems are typically used for driving wheels. The inner ring rotation systems and outer-ring rotation systems are typically used for driven wheels. Double-row angular contact ball bearings, that have desired bearing stiffness, achieve sufficient durability against misalignment. Also, they have a small rotational torque to improve fuel consumption. Thus, they are frequently used for the wheel bearing devices.
The structures of wheel bearing devices are classified broadly into four generations. A first-generation structure includes a wheel bearing with a double-row angular contact ball bearing fit between a knuckle of a suspension and a wheel hub. A second-generation structure includes a vehicle-body-mounting flange or a wheel-mounting flange directly formed on the outer circumference of an outer member. A third-generation structure includes which an inner raceway surface directly formed on the outer circumference of a wheel hub. A fourth-generation structure has inner raceway surfaces directly formed on the outer circumference of a wheel hub and the outer circumference of an outer joint member of a constant-velocity universal joint.
The structure illustrated in FIG. 3 is known as an example of the structure of such a wheel bearing device. This wheel bearing device is referred to as a third-generation wheel bearing device on a driven wheel side. It includes an inner member 53 formed from a wheel hub 51 and an inner ring 52. An outer member 55 is mounted around the inner member 53 with double-row balls 54 interposed between the two. In the following description, where the wheel bearing device is assembled on the vehicle, the outside of a vehicle is referred to as an outer side (left-hand side in FIG. 3). A central side of the vehicle is referred to as an inner side (right-hand side in FIG. 3).
The wheel hub 51 includes a wheel-mounting flange 56. The flange 56 mounts a wheel (not illustrated). The wheel mounting flange 56 is integrally formed on an outer-side end portion. Hub bolts 56a are inserted into the wheel-mounting flange 56 at regular intervals about its circumference. An outer-side inner raceway surface 51a and a cylindrical portion 51b are formed on an outer circumference on the wheel hub 51. The cylindrical portion 51b extends axially from the inner raceway surface 51a. The inner ring 52 is press-fit on the cylindrical portion 51b. The inner ring 52 has an inner-side inner raceway surface 52a formed on its outer circumference.
The outer member 55 includes a vehicle-body-mounting flange 55b that is to be mounted onto a knuckle (not illustrated). The flange 55b is integrally formed on an outer circumference of the outer member 55. Double-row outer raceway surfaces 55a are integrally formed on an inner circumference of the outer member 55. The double-row balls 54, retained by cages 57, are rollably accommodated between the outer raceway surfaces 55a and the double-row inner raceway surfaces 51a and 52a that face the outer raceway surfaces 55a. Seals 58 and 59 are attached to both end portions of the outer member 55. The seals prevent lubricating grease sealed inside the bearing, from leaking out. Additionally, the seals prevent, for example, rainwater and dust from entering into the inside of the bearing from the outside.
The outer member 55 has a slightly inclined outer circumferential surface 60 formed on the outer side. A cylindrical fitting surface 61 is formed on the inner side. The fitting surface 61 is to be fit into the knuckle. The vehicle-body-mounting flange 55b is interposed between the outer circumferential surface 60 and the fitting surface 61. A large-arc-shaped corner 62 is formed by a forging process. The corner 62 is formed at a portion where the vehicle-body-mounting flange 55b meets the outer circumferential surface 60. The outer member vehicle-body-mounting flange 55b is discontinuously formed in a circumferential direction. Protruding portions have a tapped hole 63. The outer member 55 is to be fastened to the knuckle by fastening bolts, not illustrated.
An inner-side flange surface 61a of the vehicle-body-mounting flange 55b is formed into a flat vertical surface by a machining process such as a turning process. An outer-side flange surface 60a is formed by a forging process. It has a flat surface tapered at an inclination angle θ such that the thickness (a) of a flange base 64 gradually decreases in the direction to the outside in the radial direction. This enables the weight and size of the outer member 55 to be reduced without decreasing its strength and stiffness. Also, it makes it easy for material to reach an end portion of the vehicle-body-mounting flange 55b during a forging process. Thus, forging workability and processability is improved. The amount of a burr created during the forging process can be decreased. The precision can be improved, and the lifetime of a mold can also be improved.
In the conventional outer member 55, the outer-side flange surface 60a is formed into a flat surface tapered at an inclination angle θ. Accordingly, it is difficult to drill a pilot hole to form the tapped hole 63. For this reason, as illustrated in FIG. 4, an outer-side flange surface 66 of a vehicle-body-mounting flange 65 is formed as a flat vertical surface 66a by a forging process. It extends from the flange base 64 toward the outside in the radial direction. A flat surface 66b is tapered at a predetermined inclination angle θ. It is formed such that the thickness of the vehicle-body-mounting flange 65 gradually decreases in the direction to the outside in the radial direction. Spot-facing is performed on the root of the vehicle-body-mounting flange 65 at a position corresponding to the tapped hole 63. A recessed portion 67, extending in the axial direction, is formed on the corner 62.
The recessed portion 67 is formed by a cutting process with a milling machine. Subsequently, the pilot hole is formed by a drilling process. A tapping process is performed in the pilot hole to form the tapped hole 63. This prevents processes of forming the pilot hole and the tapped hole 63 from being disturbed even in the case where the tapped hole 63 is formed such that the PCD of the tapped hole 63 is decreased and a part is connected to the recessed portion 67. Accordingly, the PCD of the tapped hole 63 can be dramatically decreased. Thus, the weight and size of the outer member can be further reduced without decreasing the strength and stiffness of the outer member. See, Japanese Patent No. 5242957.